The present invention relates to surveying methods and apparatuses for measuring coordinates of a target point. The methods and apparatuses are based on the signals of satellite radio navigation systems. The invention is especially efficient under strong multipath conditions.
Satellite navigation systems include the global positioning system (GPS) and the global orbiting navigation system (GLONASS) and are used to solve a wide variety of tasks that related to determining object position, object velocity, and precise time. Land surveying is an important application of receivers based on satellite navigation systems. Such receivers have a lot of advantages compared to the conventional devices for land surveying. In comparison to conventional surveying devices, satellite-based surveying systems are more responsive, can operate in nearly all types of weather and at all times of the day, and can be used in areas which do not have line-of-sight conditions.
Any measurement procedure is characterized by its efficiency (productivity). In the case of surveying, it is the number of point position measurements that can be made per unit of time within a predetermined accuracy. To improve efficiency, we should reduce the time duration of a single measurement. However at this, it is necessary to simultaneously increase measurement accuracy, because a reduction in the time duration of the single measurement can result in a deterioration in accuracy if special precautions are not used.
Many survey applications require sub-centimeter positioning accuracy, i.e., accuracy to within several millimeters. To achieve this, the receiver, which is often called the xe2x80x9croverxe2x80x9d, operates in phase differential mode with a base station that has a position known with high accuracy. The coordinate difference between a rover and a base station, which is called the xe2x80x9cbase vector,xe2x80x9d can be determined in this mode. For this, we use the satellite carrier phase difference between the base and rover. It can be calculated by processing data sets from the base and rover. Data sets of measurements from the base station are called differential corrections. The rover is placed on a point whose coordinates need to be ascertained, i.e., a target point, while the base station is placed on a point with precisely known coordinates, i.e., a land mark. The receiver antennas are mounted, for instance, on respective tripods.
Knowing the coordinates of the base vector and the base station, it is possible to compute the rover""s coordinates by summing the base-station and base vector coordinates together. For computing, one needs to know both the land-mark position relative to the phase center of the base antenna and the target point position relative to the phase center of the rover""s antenna, since a satellite navigation system can determine a base vector only between the phase centers of the antennas.
To simplify the transformation of the phase center position into target point position (and vice versa, landmark position into phase center position), the phase center of the antenna is usually situated vertically above the landmark or target point using a plumb bob, level vial, or other instruments. In this case, for the transformation we need to know just the difference in height between the antenna""s phase center and the corresponding landmark or target point.
However, such a procedure of vertical alignment is time-consuming, and is an acceptable burden only for the base station, not the rover. The base station is usually set up to operate for a long time, while the rover is usually fixed for a short time. For instance, such a procedure as real time kinematic (RTK) surveying usually requires that the minimum possible amount of time be used to set up the rover on target point in order to improve measurement efficiency. In practice, almost all of the time needed for an RTK surveying measurement is spent on this set-up time. As usual, in such cases one employs a range pole with a bubble level vial. The antenna is mounted on top of the range pole, and the bottom pole tip is placed on target point.
We should note that using such an instrument does not provide sub-centimeter accuracy because of the possible trembling of the operator""s hand. To reduce this trembling, one can use a bipod which has two extra legs to achieve a stable pole position, but this results in an undesirable increase in the set-up time.
An alternative way is to provide the range pole with a tilt sensor and magnetic sensor (compass) that determines direction of the tilt in the horizontal plane. When processing this sensor data, it is possible to ascertain the direction and amount of the pole""s tilt, and to then transform the position of the phase center in the target point (U.S. Pat. No. 5,512,905). However, due to their inherent errors, sensors do not allow the accuracy of transformation to better than 1 to 2 cm when using a two-meter length range pole. Moreover such a device is relatively expensive and complicated. The main source of errors for the tilt sensor is temperature drift of measurements, and for the magnetic sensor, it is both neighboring iron objects and local magnetic anomalies.
There is a possibility of doing without any of the above sensors. It is possible if we process a set of measurements obtained when swinging the pole while keeping contact of the pole tip with the target point (U.S. Pat. No. 5,929,807). As the pole length is constant, all of the measured points will be placed on a sphere with a radius equal to the pole""s length. The set of measurements can be processed with the least squares technique (LST) to determine the position of the sphere""s center, which will be the position of the target point. This approach provides high accuracy in the height of the target point. But at this, the accuracy of the plane position will be poor (not better than 3 to 4 cm) because of the limited swing angle sector of the pole (tilt angle not greater than 20 degrees). This limitation is connected with the shape of an antenna radiation pattern, because if the tilt is greater than 20 degrees, the signal power at the antenna output is too weak to reliably track satellites having low elevation angles. The limitation is also related to inconvenience for the operator to swing the pole with a greater angle since it makes him bend. Note the angle sector of 90 degrees is needed to reach better accuracy, that is, the antenna should be swept through the range of a semi-sphere. However, this is impossible. So, both considered alternative approaches for the transformation of the phase center position into the position of the target point cannot provide sub-centimeter accuracy.
Another source of coordinate errors are multipath errors arising from the reception of signal replicas along with the line-of-sight signal from the satellite. These replicas are reflected from neighboring objects and have parameters different from line-of-sight signal parameters. The total signal received by the antenna and measured by the receiver will be a combination of the parameters of the line-of-sight signal and the parameters of the multipath signals. Thus, the parameters of the total signal will be different from the parameters of the line-of-sight signal, and there will be a resulting multipath error. This error can be about 1 to 3 cm, depending upon operation conditions. In addition, multipath signals can result in anomaly errors having values much greater than the ones given above.
Under differential mode at short baselines, when baseline length is less than 10 km, the multipath error in the antenna""s phase center position becomes prevailing. In reality, using differential mode enables one to eliminate almost totally the majority of position error sources which are related to the satellites (selective availability, ionosphere delay, instability of the satellite clock, inaccuracy of ephemeris information). Error elimination is achieved by their inter-compensation at subtraction, since they are present in the same manner at both the base station and the rover. However, this cannot be said about the multipath error because it is determined by the local environment where the antenna is set up.
Multipath errors affect measurement precision in the following two ways. First, it causes an error in the base vector coordinates, and this prevents one from obtaining sub-centimeter accuracy. Such an error is determined by the carrier phase multipath. Second, the time required to reliably resolve ambiguities (an integer number of carrier wavelengths) increases. Knowing the ambiguities is necessary to compute the base vector coordinates. Not only the carrier phase multipath error but also the code pseudo-range) multipath error impact the ambiguity resolution time. This extra time is especially noticeable for single-frequency navigation receivers.
There are different ways of reducing multipath error. Smoothing methods should be mentioned first of all. Here, already computed base vector coordinates can be subjected to smoothing over a time duration much greater than a correlation interval of the multipath error. Smoothing code measurements with carrier phase can be employed to reduce multipath error on code measurements. (see xe2x80x9cUnderstanding GPS: principles and applicationsxe2x80x9d. Elliott Kaplan, Artech House, 1996, chapter 8, pp. 364-367, ISBN 0-89006-793-7). These approaches have, however, a series of limitations and disadvantages, the main one being the necessity of long measurements (ten or more minutes) due to a large correlation interval of multipath error to achieve good smoothing.
There exist techniques based on considering the behavior of in-phase component I that was obtained as a result of the correlation processing for a satellite signal. (Dai D., Walter T., Comp C., Tsai Y., Ko P., Enge P., Powell D., xe2x80x9cHigh integrity multipath mitigation techniques for ground reference stations,xe2x80x9d Proc. of the 1997 Int. Tech. Meeting of the ION, Nashville, Tenn. 1997, pp. 593-604). The fact is that a reflected signal results in I changing. Then, one can compensate for multipath error if the pseudorange, carrier phase, and component I are co-processed. The main drawbacks of this method are low accuracy and the failure to track changes of the error fast enough.
There are also methods that use building of channel algorithms (tracking systems) taking reflected signals into consideration. The systems with any specially selected shape of the reference signal in tracking system correlators are well known (see Veitsel V., Zhdanov A., Zhodzishsky M., xe2x80x9cThe mitigation of multipath errors by strobe correlators in GPS/GLONASS receiversxe2x80x9d GPS Solutions, Volume 2, Number 2, Fall 1998). Some systems employing several correlators are known as well (see Van Nee, J. Siereveld, P. Fenton and B Townsend, xe2x80x9cThe Multipath Estimating Delay Lock Loop: Approaching Theoretical Accuracy Limits,xe2x80x9d Proc. of the IEEE Position, Location and Navigation Symposium, Las Vegas, Nev., USA, 1994). In such systems a combination of the output signals of these correlators is used to track signals. The shape of the reference signal in the first case, and the number of the correlators and their combining rule in the second case are chosen to minimize impact of multipath on measurements.
The main drawback of these systems is an impossibility of multipath suppression at short delays (from 0 to 20 m) of the reflected signal compared to the line-of-sight signal. The reflected signals with such delays often arise when there are reflecting objects near the antenna, for instance, trees, cars, buildings, chimneys, towers, pillars, derricks, and other man-made objects. Such a situation can appear under operation in urban canyons or wooded terrain.
Many techniques of multipath suppression have been described which use antennas with special reception patterns that consider the influence of reflected signals. To mitigate multipath due to signal reflection from the Earth""s surface, we can utilize special screens on which the antenna can be mounted. One of the following screen types is typically used: ground-plane (the screen having shape of a flat metal disc), or choke ring (the screen as a disc with concentric rings on it). A choke ring screen provides a higher multipath suppression level than a ground-plane screen, but it inherently has greater weight and size. The main limitation of the xe2x80x9cscreenxe2x80x9d approach is a failure to suppress the reception of multipath signals which are in the upper semi-sphere above the antenna.
To mitigate the multipath signals in the upper semi-sphere above the antenna, it is possible to use a series of space-diversity antennas. The operation principle of such systems is based on the fact that multipath parameters are different in different space points. Having processed a series of signals obtained from the antennas, we can substantially reduce the multipath error. In this case, the more antennas and the more distance at which they are spaced from each other, the better the reduction is. However, overweight and greater sizes can be considered the basic faults of such a system.
Another approach is to use the multipath randomization effect, that is, the effect of averaging the multipath error by moving the antenna in space. Thus, for an antenna mounted on the movable vehicle (e.g., a car), the accuracy of the pseudorange measurements is greatly increased if code observables are smoothed by carrier phase observables (see xe2x80x9cMitigation of multipath in DGPS ground reference stationsxe2x80x9d, by M. S. Braasch, F. van Graas, Proc. of the National Technical Meeting, The Institute of Navigation, San Diego, Calif. Jan 27-29 1992). To obtain the randomization effect for static measuring systems, it is necessary to move the antenna along any determinate closed loop, e.g., a circle, by a mechanical driver (see xe2x80x9cGPS multipath mitigation by antenna movements,xe2x80x9d B. J. H. van den Brekel, D. J. R. van Nee, Electronics Letter, Dec. 3, 1992, Vol 28, No.25xe2x80x94In this paper the driver represents a xe2x80x9crotating handxe2x80x9d). The multipath errors in the pseudorange will be averaged with a narrow bandwidth delay lock loop (DLL). The main drawback of this approach is an insufficient level of multipath mitigation (one cannot use a DLL with too narrow of a bandwidth, otherwise there will be a loss of signal tracking). A secondary drawback is the large weight and size of the antenna movement system.
An objective of the present invention is to develop methods and apparatuses that provide improvement of surveying measurement efficiency by increasing accuracy to the sub-centimeter level and reducing the time of coordinate determination for a target point with satellite navigation systems using more complete suppression (i.e., reduction) of multipath during a short time interval. Another objective of the present invention is to improve the efficiency of the measurement process by providing simultaneous, fast and accurate fixing of a target point related to the phase center of an antenna.
Broadly stated, the present invention encompasses methods and apparatuses for estimating one or more coordinates of a target point from one or more measurement sets made from a satellite navigation antenna which is mounted to one end of a pole, or other mechanical structural member, with the other end of the pole (or structural member) in contact with the target point. Each measurement set comprises one or more measure antenna coordinates, which may be generated at a given time moment by a global-positioning satellite receiver, and a corresponding value representing the inclination angle of the pole (or structural member) relative to the plumb position axis at the given time moment. The plurality of measurement sets preferably are made with the antenna positioned at different locations around the target point.
One exemplary apparatus embodiment of the present invention comprises an antenna, a navigation receiver which receives differential corrections from a base station, a range pole, and a tilt sensor which measures the angle of the pole axis relative to the plumb-position axis, the latter being collinear with the direction of gravitational pull at the target point. The tilt sensor is placed into a housing which is attached to the pole or which serves as the pole""s tip at the pole""s first end. The antenna is mounted at the top end of a range pole (its second end) and provides its output to the navigation receiver. The navigation receiver, with the aid of corrections from the base stations, generates measured antenna coordinates for the antenna""s phase center which are provided to a data processor. The inclination data from the tilt sensor is also provided to the data processor, and the data processor generates estimates of the target point given a plurality of measured antenna coordinates and inclination angles. In use, an operator places a pole tip on a target point and swings the range pole by hand in different directions over an angle sector of at least 5 degrees relative to the plumb-position axis, and preferably over an angle sector of 15 degrees. As an option, the operator may also rotate the antenna about the axis of the pole. During the swinging operation, the system collects data related to the antenna""s position and the inclination angle of the pole. Based on this data and the distance from antenna""s phase center to the pole tip (at the target point), the present invention can estimate the height of the target point with one measurement set, and the two planar coordinates with three measurement sets. In typical implementations, each measurement set can be generated in less than 5 seconds, and usually in one second or less. In moving the antenna in this manner, the multipath error in navigation data for different measurement sets is almost uncorrelated, and both the time of ambiguity resolution for carrier phase and coordinate position error of the target point will be decreased. There is no need to vertically align the range pole.
It is possible to generate estimates for the target point coordinates both in post-processing and real time. In the first case, from the beginning one records measurements for the rover and base station, and then performs their processing at a subsequent time. In the second case, a system comprises a radio-modem to receive differential corrections from the base station. The measurement sets may be generated at a periodic rate, and the rover may be configured to iteratively compute the coordinates of the target point in real time as each new measurement set is generated. The accuracy of coordinate computations gradually increases with each new set.
There are a number of advantages to this embodiment. The inclinometer is a small, low-cost unit adapted for use with any conventional range pole and exchanges data with the receiver according to a standard protocol. Such a system is easy to operate and provides fixing of the coordinates of the target point with improved accuracy and reduced measurement time, especially in a multipath environment. This system does not require vertical alignment of the range pole as well.